1. Field of the Invention
The present invention relates to a lithographic apparatus, a patterning device for use with such a lithographic apparatus, and a method for determining higher-order distortions of a patterning device of a lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The substrate (e.g. a wafer) is usually supported by a wafer stage. By moving the wafer stage, the substrate can be positioned relative to the reticle. In present lithographic apparatus, a device for transmission image detection is used in order to align a reticle to a wafer stage. Said device comprises a structure (e.g. a grating) on a reticle and a complementary structure on a transmission image detector plate of a transmission image detector. By transmitting a radiation beam through the structure on the reticle and detecting an image of said structure by the transmission image detector on the wafer stage the position and focus of the image can be determined. In practice, the transmission image detector comprises several structures. Beneath each structure a photodiode is located to detect the transmitted radiation beam. The transmission image detector is conventionally used to measure first-order positioning terms like translation, magnification and rotation of the reticle with respect to the wafer stage, so that the transmission image detector is able to measure first-order reticle distortions.
However, higher-order reticle distortions remain unresolved and any reticle distortion, including these higher-order reticle distortions, may lead to image distortions of the pattern on the substrate resulting in overlay errors and focus errors.
Overlay is an important factor in the yield, i.e. the percentage of correctly manufactured devices. Overlay is the accuracy within which layers are printed in relation to layers that have previously been formed. The overlay error budget will often be 10 nm or less, and to achieve such accuracy, the substrate must be aligned to the pattern on the reticle with great accuracy.
Reticle distortions may have many different causes. To name a few, distortions may arise as a consequence of:                reticle writing errors;        reticle heating;        holding the reticle in a reticle support (clamping effects and gravitational sag);        reticle fabrication;        assembling the reticle with other devices, such as a so-called pellicle.        
A pellicle is a device that prevents dust particles from landing on the reticle as it is easier and less risky to replace a “dirty” pellicle than to clean a “dirty” reticle, and keeps dust particles at an out-of-focus distance from the reticle, so that their influence on the pattern to be imaged on a wafer is minimal. A pellicle usually comprises a pellicle membrane which is transparent to the radiation beam, a pellicle frame, and an adhesive to attach the pellicle to the reticle. The pellicle is attached to the reticle such that it covers the pattern on the reticle and shields that part of the reticle from the rest of the lithographic apparatus.
Because of the rigid nature of the pellicle frame, the reticle will experience mechanical distortions upon placement of the pellicle, resulting in higher-order image distortion. Furthermore, when the reticle is subsequently placed, i.e. chucked, on a reticle support to hold the reticle, this distortion becomes worse by gravitational sag and chucking stress.
During the fabrication of a reticle several distortions may be introduced, which will be elucidated here. When a reticle blank or substrate is fabricated, it is not perfectly flat, but it will have a concave shape, a convex shape, or an even more complex shape. This non-flat shape will introduce distortions to the image. The reticle blank is further coated with one or more (absorber) layers and a resist. The stress in these layers may have an impact on the shape of the reticle. Subsequently, an image of the pattern which has to be transferred to the reticle is transferred to the resist on the reticle by a reticle writing tool. This can be done by means of laser radiation or charged particle (e.g. electron) radiation. Irradiation of the reticle can result in local heating effects and distort the reticle. The use of the charged particle radiation (e.g. an e-beam tool) can result in charging effects and distort the image during writing. Subsequently, the resist is processed and this processing can introduce further distortions of the image in the resist. The image in the resist is then transferred into the underlying layer (and if required into the substrate). The underlying (absorber) layer or layers (and if required the substrate) are locally removed or etched. This can introduce distortion of the image in the underlying layer or layer(and if required the substrate). Removal of material in the layer or layers (and if required the substrate) can result in relaxation of stress that was build up earlier by the deposition of the (absorber) layer(s) and the resist. As a consequence, the reticle can experience distortion of the image.
When irradiating a reticle, some areas will absorb the radiation, and other parts will transmit the radiation, thereby forming a patterned radiation beam. However, absorbing the radiation will result in a local increase in temperature, thereby introducing distortions to the image field.
The pellicle induced distortions, reticle fabrication induced distortions and the chucking induced distortions have a static, or quasi-static nature, i.e. there magnitude is relatively constant over time. As a pellicle may be replaced by another pellicle, the pellicle induced distortion may change abruptly due to this replacement.
The distortions induced by heating have a dynamic nature and are dependent amongst others on the pattern on the reticle. The more radiation that is absorbed by the pattern, the more the reticle will deform due to heating.
It has been shown that especially for high-throughput lithographic apparatus, the higher-order distortions have a large contribution in the overlay budget.